Although the built-in light metering of cameras in the digital era is good, the external light metering meter still has value, and is a great weapon for film lovers. It is not a new thing to use smart phones as light meters, but Lumu, a combination of hardware and software, is a good choice. The hardware part of Lumu is a light bulb, which uses the headphone jack to exchange data with the mobile phone, feel the ambient light illumination, and then